Internal combustion engines may operate in a variety of combustion modes. One example mode is homogeneous charge compression ignition (HCCI), wherein an air and fuel mixture achieves a temperature where combustion occurs by auto-ignition without requiring a spark by a sparking device.
Effective and stable HCCI combustion timing can be achieved by controlling in-cylinder temperature and pressure within prescribed limits. However, HCCI combustion may be sensitive to air and cylinder temperature variations. Further, resulting cylinder-to-cylinder temperature imbalances and variability can lead to inefficient combustion, or even misfires. One approach for more accurately controlling combustion timing is described in U.S. Pat. No. 7,128,048 B2 wherein a cylinder-specific combustion state is estimated and adjusted on a cylinder basis.
In U.S. Pat. No. 7,128,048 B2, by controlling a variable valve mechanism, a fuel injection mechanism, and a transmission, either independently or coordinately, based on input received from a combustion state estimating means, the method controls combustion timing. Specifically, the combustion state estimating means estimates the combustion state of each cylinder by detecting the cylinder pressure peak value and peak timing and assessing the deviation of these values from a predetermined range of values. When regulation of an individual cylinder's peak pressure value or timing in relation to a predetermined value or timing is brought about via the fuel injection mechanism, the combustion control means adjusts the amount of fuel injected to that cylinder during the minus overlap period.
However, the inventors herein have recognized disadvantages with such an approach. For example, the fuel injected during negative overlap, also known as the pilot injection, is burned with very low efficiency in terms of torque production due to a limited compression and expansion of the gas. This results in a fuel economy penalty which reduces the benefits of the HCCI engine combustion. Another disadvantage is that while the amount of fuel injected into a cylinder is adjusted to affect that cylinder's combustion timing, the torque produced by that cylinder may also be affected. As such, while combustion timing of that cylinder may be brought closer to that of the remaining cylinders, the combustion torque of that cylinder may diverge away from that of the remaining cylinders.
The inventors herein have recognized that a cylinder-by-cylinder balancing can be achieved with a main injection, while maintaining the average engine torque during the cycle at a desired amount. This method may be applied regardless of whether a negative-overlap HCCI control mechanism or, as specified later in this disclosure, a heated intake-air HCCI control mechanism is used. Thus, in one example, the above issues may be addressed by a method of operating an engine in a vehicle carrying out homogeneous charge compression ignition by controlling fuel injection to a plurality of cylinders, the method comprising: adjusting a fuel injection amount in a first cylinder to adjust a combustion timing of the first cylinder in a first direction, and correspondingly adjusting a fuel injection amount in the remaining cylinders in a second direction, such that average engine torque is maintained and timing imbalance is reduced. For example, when the first cylinder and second cylinder have a difference in combustion timing (e.g., either the first cylinder is too advanced, or the second cylinder is too retarded), the fuel injected to both of the first and second cylinders is adjusted, albeit in different directions.
In this way, by adjusting the fuel in each cylinder in different directions, both cylinders can be brought closer to a common combustion timing, while also maintaining the average torque produced by the two cylinders. Thus, cylinder-by-cylinder combustion timing and average torque control may be achieved.